The present invention is directed to an elapsed time indicator, and more particularly, an elapsed time indicator utilizing electrophoretic effects to move suspended pigment particles in an electric field in times certain.
Elaspsed time indicators have been contemplated using electroplating effects. Such devices have been utilized in vidicon tubes, recording heads and klystrons to indicate elapsed time of operation for this equipment. Such prior devices are current dependent, and must utilize rated resistors to obtain the proper current for the desired time interval from a specified DC voltage.
Moreover, prior elapsed time indicators operate on an irreversible electrolytic deposition techniques, such as those described in U.S. Pat. No. 2,970,264. As such, irreversible type elapsed time indicators are costly inasmuch as they are not reusable, and require extensive circuitry for current control.
The present invention seeks to avoid these difficulties in the prior art by providing an elapsed time indicator operating under principles of electrophoresis. Principally, such electrophoretic elapsed time indicators are voltage dependent, requiring only simple voltage regulators for proper operation, are reversible, and can be utilized for any desired elapsed time by simply providing the proper applied voltage.
These effects may be accomplished according to the present invention by suspending very fine pigment particles in an organic liquid between two electrodes. Electrophoresis is an electrokinetic effect wherein solid particles move in a liquid upon the application of a potential gradient across the liquid. In accordance with the elapsed time indicator of the present invention, the pigment particles may be of a given color, while the liquid suspension may be dyed of an opposite color. For example, the pigment particles may be light in color while a dark color dye may be added to the liquid suspension.
Accordingly, upon the application of a voltage across the electrophoretic suspension, the light color pigment particles will be attracted to an electrode of opposite polarity. When the polarity on the electrodes is reversed, the light colored pigments will be caused to move in the liquid to the opposite electrode, thereby resulting in a change of color as the light colored pigments move toward the opposite electrode.
If the electrophoretic suspension is within a small diameter tube having electrodes at each end, then the movement of the light colored pigment particles from one end of the tube to the opposite end will result in a light colored bar being transmitted the length of the tubing. Since the movement of the pigment particles occurs in a transist time dependant upon the applied voltage, the distance between the electrodes, and the electrophoretic mobility, then the position of the light bar in the tube provides an indication of elapsed time.
Thus, it can be shown that the electrophoretic transit time t may be given by the relationship EQU t=d/v (1)
where d is the distance and v is the velocity
The particle velocity is the product of the electrophoretic mobility u and the electric field V/d where V is the applied voltage. Accordingly, the article velocity becomes EQU v=u(V/d) (2)
The electrophoretic mobility of spherical particles is given by the following relationship EQU u.epsilon..zeta./6.pi..eta. (3)
Where .epsilon. and .eta. are the dielectric constant and viscosity, respectively, of the suspending medium, and .zeta. is the zeta potential. Combining equations (2) and (3) into equation (1), it is apparent that the transit time is proportional to the viscosity and to the square of the distance between the electrodes, and inversely proportional to the applied voltage, the dielectric constant, and the zeta potential. EQU t=6.pi.d.sup.2 .eta./V.epsilon..zeta. (4)
Accordingly, the distance between the electrodes and the applied voltage are two parameters for controlling the transit time, which can be easily controlled. Of the two, the distance between the electrodes provides the most dramatic change.